md/raid1: handle flush request correctly

I got a warning triggered in align_to_barrier_unit_end. It's a flush
request so sectors == 0. The flush request happens to work well without
the new barrier patch, but we'd better handle it explictly.

Cc: NeilBrown <neilb@suse.com>
Acked-by: NColy Li <colyli@suse.de>
Signed-off-by: NShaohua Li <shli@fb.com>

drivers/md/raid1.c

@@ -1282,8 +1282,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio)
 	unsigned long flags;
 	const int op = bio_op(bio);
 	const unsigned long do_sync = (bio->bi_opf & REQ_SYNC);
-	const unsigned long do_flush_fua = (bio->bi_opf &
-						(REQ_PREFLUSH | REQ_FUA));
+	const unsigned long do_fua = (bio->bi_opf & REQ_FUA);
 	struct md_rdev *blocked_rdev;
 	struct blk_plug_cb *cb;
 	struct raid1_plug_cb *plug = NULL;
@@ -1509,7 +1508,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio)
 				   conf->mirrors[i].rdev->data_offset);
 		mbio->bi_bdev = conf->mirrors[i].rdev->bdev;
 		mbio->bi_end_io	= raid1_end_write_request;
-		bio_set_op_attrs(mbio, op, do_flush_fua | do_sync);
+		bio_set_op_attrs(mbio, op, do_fua | do_sync);
 		if (test_bit(FailFast, &conf->mirrors[i].rdev->flags) &&
 		    !test_bit(WriteMostly, &conf->mirrors[i].rdev->flags) &&
 		    conf->raid_disks - mddev->degraded > 1)
@@ -1565,6 +1564,11 @@ static void raid1_make_request(struct mddev *mddev, struct bio *bio)
 	struct bio *split;
 	sector_t sectors;
 
+	if (unlikely(bio->bi_opf & REQ_PREFLUSH)) {
+		md_flush_request(mddev, bio);
+		return;
+	}
+
 	/* if bio exceeds barrier unit boundary, split it */
 	do {
 		sectors = align_to_barrier_unit_end(